"However, our study found that the primarily cognitive secondary task of talking on a hands-free device does not appear to have any detrimental effects," said Tom Dingus, director of VTTI and the principal investigator of the study.

Using video and other sensor data, the team analysed video footage of 3,454 drivers, 905 crashes (including 275 more serious crashes), and 19,732 control periods of "normal driving" for instances of cognitive distraction.

For comparison, they also studied examples of drivers performing visual and manual activities, such as texting on a hand-held phone or adjusting the radio.

"There are a number of reasons why using a hands-free device could keep drivers more engaged and focused in certain situations," said Dingus. "One is that the driver looks forward more during the conversation".

Although engaging in the conversation could cause a small amount of delay in cognitive processing, the driver is still more likely be looking in the direction of a precipitating event, such as another car stopping or darting in front suddenly.

"The phone conversation could also serve as a countermeasure to fatigue on longer road trips. Perhaps most importantly, a driver who is talking on a hands-free phone is less likely to engage in manual texting/browsing/dialing and other much higher-risk behaviours," Dingus added.